Can Pregnancy Change Your Bloodtype?

My mom had her blood typed recently and was shown to be B+. Her medical records from 50 years ago say she was B-. My sister says pregnancy can change a woman's blood type. My sister says she also went from B- to B+. Is this possible?

your blood type does not change. Labs test more throughly now than they did years ago. I was classified as ab- but I had to receive blood a few months ago and I had to take ab+.A lab could explain it to your mother.